The present invention relates to programmable dividers. More particularly, it is concerned with an improved programmable divider for use in a frequency synthesizer which may be employed in an FM broadcast receiver.
Broadcast receivers for frequency modulated signals are generally of the superheterodyne type. Such receivers must have some means for stabilizing the local oscillator frequency against frequency drifting which can cause distortion of the received signal.
Automatic frequency control (AFC) systems use the received signal to stabilize the local oscillator frequency. However, AFC systems are ineffective when the received signal is weak or transmission on channels adjacent the received signal channel are particularly strong.
Another possible solution is to make the local oscillator so stable that an automatic frequency control systems becomes unnecessary; however, the cost of building a local oscillator with the required degree of stability is so high as to make this approach an undesirable one.
A third solution is to utilize a frequency synthesizer employing a phase-locked loop. In such a loop, the local oscillator signal is divided-down by an integer value to produce a submultiple frequency which nominally matches the frequency and phase of a fixed reference frequency output from a stable crystal oscillator. The programmable dividers which divide-down the local oscillator output are digital logic devices which have required a digital input to select the tuned channel.
In more expensive communications equipment, the input to the programmable divider may be provided by thumb wheel switches, by keyboard data entry, or from the memory banks of a digital computer. These techniques for generating the divider input are generally not suited for entertainment quality FM broadcast receivers since they are too expensive and/or require the user to remember exact station frequencies. Many users have no desired to remember station frequencies and instead prefer to tune in stations simply by manipulating the tuning knob until the desired signal is received.
Since the signals provided by rotary tuning knobs are in analog form while the known programmable dividers have accepted only digital inputs, it has been thought to be necessary to provide an analog-to-digital conversion. Unfortunately, the componentry required for an analog-to-digital conversion is relatively complex and costly, making such a receiver possibly as expensive as a receiver having a direct digital input to the programmable divider.